Contamination of the environment by toxic heavy metal

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1 TECHNICAL REPORTS: TECHNICAL HEAVY METALS REPORTS IN THE ENVIRONMENT Sorption of Lead from Aqueous Solutions by Tea Wastes Ni Liu, Daohui Lin,* Huifeng Lu, Yong Xu, Miaolong Wu, and Jin Luo Zhejiang University Baoshan Xing University of Massachusetts Environmental contamination by heavy metals has long been a worldwide concern. Tea wastes, having porous surfaces with polar functional groups, could be a good sorbent for removal of Pb(II) from wastewaters. This study aimed to investigate the potential of tea wastes as a sorbent for removal of Pb(II) from solution and the underlying sorption mechanism. Tea wastes showed high removal efficiency for Pb(II) with a short equilibration time and high sorption capacity. The sorptive affinity increased with increasing solution ph and leveled off at about ph 5. Higher temperature led to a higher sorptive affinity, indicating the sorption being an endothermic process. Coexisting metal ions lowered the sorption of Pb(II) with an order of Ag(I) < Cu(II) < Al(III). Fourier transform infrared (FTIR) spectrometer and scanning electron microscopy (SEM) with an X-ray energy dispersion spectroscopy (EDS) accessory were used to examine the underlying mechanism of the Pb(II) sorption. Surface complex formation with carboxylic and amine groups and ion exchanges were observed to regulate the binding of Pb(II) to the tea wastes. Copyright 2009 by the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Contamination of the environment by toxic heavy metal ions through the discharge of industrial wastewaters has long been a worldwide concern (Amarasinghe and Williams, 2007). Among the toxic heavy metals, lead as well as copper, cadmium, chromium, and mercury presents high potential danger to human health (Bailey et al., 1999; Doyurum and Celik, 2006). Lead ion [Pb(II)] concentration in industrial wastewaters can be up to 200 to 500 mg L 1 (Li et al., 2008), which is very high in terms of water quality standards of 0.05 to 0.1 mg L 1. Therefore, highly efficient removal of Pb(II) is an important issue for protection of aquatic ecosystems and human health. Coagulation with flocculants followed by precipitation is a conventional technique and has been extensively employed for metal ion removal from wastewaters. However, this process usually produces large volumes of sludge containing heavy metals (Amarasinghe and Williams, 2007). Other removal techniques include membrane filtration, sorption, reverse osmosis, solvent extraction, and membrane separation (Doyurum and Celik, 2006), among which sorption is an efficient method. Activated carbon is the widely used sorbent. However, the high cost of activated carbon limits its use in wastewater treatment. Low-cost and efficient sorbents for treatment of metal contaminated wastewaters are therefore needed. Natural materials that are available in large quantities, or certain waste products from industrial or agricultural operations may have potential as inexpensive sorbents (Bailey et al., 1999). The efficiencies of various natural sorbents in metal ion removal are attracting researcher s attentions and are being investigated. These include sugar beet (Beta vulgaris L.) pulp (Reddad et al., 2002), brown seaweed biomass (Yun et al., 2001), biomass of freshwater macrophytes (Schneider and Rubio, 1999), seaweed (Chen and Yang, 2006), wood barks (Bulut and Baysal, 2006), wood fibers (Huang et al., 2006), and sawdust (Shukla et al., 2002). Tea is the most widely consumed beverage in the world. Global tea production in 2007 was 3.60 million tonnes, of which nearly 30.6% was produced in China (Mei et al., 2009). With such a great production and consumption, large quantities of tea wastes (from the caff, cafeteria, or tea-processing factory) are usually discarded into the environment without any treatment. Furthermore, additional amount of tea leaves enters the environment by defoliation and/or pruning annually. These tea wastes could cause environmental hygiene problems during their Published in J. Environ. Qual. 38: (2009). doi: /jeq Published online 11 Sept Received 27 Mar *Corresponding author (lindaohui@zju.edu.cn). ASA, CSSA, SSSA 677 S. Segoe Rd., Madison, WI USA N. Liu, D. Lin, H. Lu, Y. Xu, M. Wu, and J. Luo, Dep. of Environmental Science, Zhejiang Univ., Hangzhou , China. B. Xing, Dep. of Plant, Soil and Insect Sciences, Univ. of Massachusetts, Amherst, MA Abbreviations: EDS, energy dispersion spectroscopy; FTIR, Fourier transform infrared spectrometer; NMR, nuclear magnetic resonance; SEM, scanning electron microscopy. 2260

2 degradation process and contaminate water environment by releasing dissolved organic matter. Therefore, reuse of these tea wastes could not only benefit the farmers but also avoid the potential environmental pollution by the tea wastes. Tea leaves contain many aromatic and aliphatic compositions with various polar functional groups, for example, OH, COOH, and NH 2 (Lin et al., 2007), and thus could be a good sorbent for contaminants. Our previous study showed the high efficiency of various tea wastes for removal of phenanthrene from the aqueous phase (Lin et al., 2007). Limited studies reported the great potential of tea leaves as a low-cost sorbent for removal of heavy metals from waste streams (Çay et al., 2004; Tee and Khan, 1988). However, the mechanism of metal binding to tea leaves is not clearly understood, and the optimal condition for metal removal still needs further experimental data to establish. This study was to assess the potential of tea wastes as a sorbent for removal of Pb(II) from wastewaters. The effects of contact time, sorbate concentration, tea variety, ph, temperature, and coexisting metal ions on the removal efficiency were investigated. The FTIR spectrometer and SEM were used to analyze the Pb(II) binding to the tea wastes. We believe that this systematic study will help to understand the underlying mechanism of the interaction between tea wastes and metal ions and to establish the optimal condition for the potential application of tea wastes for heavy metal removal from waste waters/streams. Materials and Methods Materials Six types of tea leaves were collected from tea fields in different provinces of China. They were both tender and mature leaves of Longjing tea from Zhejiang province, Yunnan largeleaf tea from Yunnan province, and Huxiang tea from Hunan province. The sample preparing procedures were detailed in our previous paper (Lin et al., 2007) and briefly described as below. The collected fresh leaves were thoroughly washed, dried, and sequentially immersed into boiling distilled water twice. The resultant subsamples were washed with deionized water, dried at 70 C, and ground into fine powders (< 1 mm) and labeled as B11 and B12, B21 and B22, and B31 and B32 for the brewed tender and mature leaves of Longjing tea, Yunnan large-leaf tea, and Huxiang tea, respectively. All of the subsamples were maintained in desiccators as sorbents for subsequent uses. The chemical reagents used such as AgNO 3, Pb(NO 3, Al(NO 3 ) 3 9H 2 O and Cu(NO 3 were all analytical reagent grade. The stock standard solution of 1000 mg L 1 of Pb(NO 3 was from Merck (Germany). Characterization of Tea Sorbents Characterization of the tea sorbents, such as elemental compositions, nuclear magnetic resonance (NMR), and FTIR analysis, had been performed in our previous study (Lin et al., 2007). Elemental contents were determined using a Vario ELIII elemental analyzer (Elementar, Germany). Solid-state cross-polarization magic angle spinning and total sideband suppression 13 C NMR spectra (CPMASTOSS) were obtained with a Bruker DSX-300 spectrometer (Karlsruhe, Germany) operated at the 13 C frequency of 75 MHz. Within the 0 to 220 ppm chemical shift range, structural carbon assignments were as follows: aliphatic C (0 109 ppm), aromatic C ( ppm), and polar C ( and ppm). The FTIR analysis was performed using a PerkinElmer Spectrum One Spectrometer with a PerkinElmer universal attenuated total reflectance (ATR) sampling accessory (Wellesley, MA). Specific surface areas of the six tea leaf powders were calculated from the adsorption desorption isotherm of N 2 at 77 K by multipoint BET method (Lin and Xing, 2008), with a surface area and pore size analyzer (Quantachrome NOVA 2000e, America). Sorption Experiments Batch sorption experiments were performed to study the removal efficiency of tea wastes for Pb(II). Five milligrams of the tea sorbents were added into 8 ml Pb(NO 3 solutions. The mixtures were shaken in a thermostat shaker (140 r min 1 ), followed by centrifugation (3000 r min 1, 20 min). The resultant supernatants of the mixtures were sampled for metal analysis by an atomic absorption spectrophotometer (PerkinElmer) using an air-acetylene flame. To determine the optimum condition for Pb(II) removal by the sorbents, the effects of contact time, sorbate concentration, tea variety, ph, temperature, and coexisting heavy metal ions on the removal efficiency were investigated. The shaker was set up at 25 C for the sorption experiments except for the temperature experiments in which 15, 25, and 50 C were used, respectively. To obtain the optimal contact time, the sorption system [mixtures of 5 mg sorbents and 8 ml 60 mg L 1 Pb(II) solution] was shaken for 5, 10, 20, 40, and 60 min, and 3, 6, and 12 h, respectively, and the removal rate [the reduced Pb(II) concentration to the initial concentration] against the contact time was analyzed. A 3-h equilibration was selected. Initial Pb(II) concentrations were 0, 5, 10, 20, 40, 60, 80, 100, 150, 200, 250, 300, and 400 mg L 1 for the experiment investigating the effect of sorbate concentration. Six types of tea leaves were used in this experiment to examine the effect of tea variety on the sorption. To study the ph effect, the solution with an initial Pb(II) concentration of 100 mg L 1 was adjusted to various ph values, while the other sorption experiments were conducted without ph adjustment (within ph 4 5). The ph of the solutions was measured using a digital Mettler Toledo ph meter. Ag(I), Cu(II), and Al(III) ions from their nitrates were examined as coexisting metal ions, respectively, for competitive sorption with 100 mg L 1 Pb(II). All the experiments in this work were run in duplicate with mean values reported. Tea sorbent (B22) after sorption of 60 mg L 1 Pb(II) for 0 min, 5 min and 3 h, respectively, and after sorption of Pb(II) (3 h) with initial concentration of 0, 10, 60, 100 mg L 1, respectively, was taken for FTIR-ATR analysis. Before the FTIR-ATR analysis, the sorbents after sorption were thoroughly washed with deionized water and dried (70 C). The FTIR spectra of pristine and Pb(II) sorbed tea sorbents in the range of 500 to 4000 cm 1 were obtained for analyzing which functional groups in the sorbents were responsible for the Pb(II) sorption. The surface morphology of tea sorbents before and after sorption of Pb(II) was examined by a Field Emission SEM (SIRION-100, FEI Corp., New York). Before SEM examina- Liu et al.: Lead Sorption from Aqueous Solutions by Tea Wastes 2261

3 tion, the samples were coated by a layer of gold to enhance conductivity. Surface elemental compositions of the samples were analyzed with an X-ray energy dispersion spectroscopy (EDS) accessory (GENESIS 4000, EDAX Inc., New York). Results and Discussion Characterization of the Tea Sorbents The elemental compositions, 13 C NMR results and FTIR-ATR spectra of the tea sorbents were detailed in our previous paper (Lin et al., 2007) and summarized in Table 1. Specific surface areas of the mature leaves (8.32, 7.97, and 5.70 m 2 g 1 for B12, B22, and B32, respectively) were about twice of the corresponding tender ones (4.09, 4.48, and 2.83 m 2 g 1 for B11, B21, and B31, respectively). The N contents in the mature leaves (2.36, 2.46, and 3.09% for B11, B21, and B31, respectively) were about two times lower than that in the corresponding tender ones (5.18, 5.18, and 5.86% for B11, B21, and B31, respectively). The mature leaves had higher H/C ratios than the tender ones, indicating that the mature leaves may contain more aliphatic carbons. 13 C NMR results further showed that the tender leaves, especially the Longjing tea variety, had higher content of polar carbons. The presence of plenty of polyphenols in the sorbents was confirmed by the 13 C NMR and FTIR-ATR characterizations (Lin et al., 2007). Effect of Contact Time The effect of contact time on Pb(II) removal by B22 is shown in Fig. 1. The sorption sharply increased in the very early stage and reached apparent equilibrium within 3 h. This sorption kinetics and equilibrium time are comparable to the reported studies on the sorptions of heavy metals to tea wastes (Ahluwalia and Goyal, 2005; Amarasinghe and Williams, 2007; Çay et al., 2004). The sorption within the first 5 min accounted for about 70% of the total removal of Pb(II) during the tested 12 h. Such a fast sorption process suggests that the tea sorbents would be applicable for practical wastewater treatment. Effect of Sorbate Concentration and Tea Variety Sorption of Pb(II) by the sorbents increased with increasing initial Pb(II) concentration, with the sorption isotherms being markedly nonlinear (Fig. 2). Thus, the most widely used Langmuir and Freundlich models were applied to fit the sorption data. They are expressed by Eq. [1] and [2], respectively. Table 1. Characterization of the tea sorbents. QbCe qe = 1 + bc [1] e q e = K f C e 1/n [2] where q e (g kg 1 ) is the amount of solute sorbed per unit weight of sorbent at equilibrium, Q (g kg 1 ) is the sorption capacity of sorbent for the solute, C e (mg L 1 ) is the liquid-phase equilibrium solute concentration, b is the sorption coefficient, and K f [(g kg 1 )/(mg L 1 ) 1/n ] and n are the constants related to sorption capacity and nonlinearity, respectively. As shown by the model fitting results (the regression coefficient R 2 ) in Table 2, the two models fit the sorption data well. The sorption capacities (Q) of the six sorbents obtained using Langmuir model were 36.5 to 81.4 g kg 1, which are comparable to the recorded sorption capacities of some biosorbents and activated carbons for Pb(II) (Table 3), indicating a good potential for Pb(II) removal from solution by the tea wastes. The sorption capacities of the mature tea sorbents of Yunnan large-leaf tea and Huxiang tea were about 1.4 and 1.6 times higher than that of their corresponding tender ones; while the specific surface areas of the mature Yunnan large-leaf tea and Huxiang tea leaves were about two times (1.8 and 2.0 times, respectively) higher than the corresponding tender ones. Furthermore, the sorption capacity of the tender Longjing tea sorbent was even 14% (10 g kg 1 ) higher than the mature one. The inconsistent order between the sorption capacities and the specific surface areas suggests that sorption could not be only determined by the surface area; chemical composition of the tea sorbents may play an important role. The tender tea sorbents, especially of Longjing tea variety, had higher contents of polar carbons and N (perhaps from amino acids) than the mature one (Table 1), which may dominate the binding of Pb(II) to the sorbent surface. Fourier Transform Infrared Spectrometer Spectrum Analysis The FTIR spectra (Fig. 3) of pristine and Pb(II) sorbed tea sorbents in the range of 500 to 4000 cm 1 were taken for analyzing which functional groups in the sorbents were responsible for the Pb(II) sorption. There is no major difference in the spectra of the sorbents after sorption of Pb(II) for different durations (Fig. 3A) and at different concentrations (Fig. 3B). However, the spectra of the sorbents after sorption of Pb(II) visually differ from that of the pristine sorbent. The peak at 2358 cm 1 corresponds to the amine (NH) Tea sample Surface area C H N O Ash H/C (O+N)/C Aliphatic C Aromatic C Polar C m 2 g 1 % % B B B B B B Data are taken and summarized from Lin et al. (2007). Surface area was measured in this study. Dry weight-based C, H, and N contents were determined using an elemental analyzer with the oxygen content calculated by mass difference. Ash contents were measured by heating the tea leaf samples at 800 C for 4 h. The data of H/C and (O+N)/C are the number ratios of the corresponding elements. The contents of aliphatic C, aromatic C, and polar C were obtained from the integration of NMR spectra in the chemical shift range of ppm, ppm, and and ppm, respectively Journal of Environmental Quality Volume 38 November December 2009

4 Fig. 1. Effect of contact time on the removal of Pb(II) (60 mg L 1 ) by the tea sorbent (B22, 625 mg L 1 ). Removal rate is the ratio of the reduced Pb(II) concentration to the initial Pb(II) concentration. group (Ahluwalia and Goyal, 2005). After sorption of Pb(II), this peak disappeared, indicating the involvement of the amine group in the sorption of Pb(II). The Pb(II) sorption slightly weakened the peak at 1622 cm 1, which is assigned to C = C vibrations in aromatic region (Lin et al., 2007), while enhanced the double peaks at 2918 and 2850 cm 1, which are assigned to asymmetric C-H and symmetric C-H bands, respectively, suggesting both aromatic and aliphatic carbons involved in the binding of Pb(II). The strong peak at 1022 cm 1 belongs to C-O stretching of polysaccharides, together with the peak around 1732 cm 1 (carbonyl stretching of -COOH group), indicating the presence of carboxylic acids. A significant increase of the peak at 1732 cm 1 and a slight decrease of the peak at 1022 cm 1 can be observed after the sorption of Pb(II), indicating the carboxylic group may play an important role in the binding of Pb(II) as well. Therefore, waste tea, especially tender ones, can be an effective sorbent for heavy metals because of its high contents of carboxylic carbon and amine groups. Table 3. Recorded sorption capacities of some biosorbents for Pb(II). Sorbent Sorption capacities Reference g kg 1 Black oak bark Bulut and Baysal, 2006 Redwood bark 6.8 Bulut and Baysal, 2006 Rice hulls 11.4 Bulut and Baysal, 2006 Sugar beet pulp 29 Reddad et al., 2002 Freshwater macrophytes 141 Schneider and Rubio, 1999 Sawdust 3 Shukla et al., 2002 Tea waste 65 Amarasinghe and Williams, 2007 Tea waste 2.1 Ahluwalia and Goyal, 2005 Tea waste This study Rice husk 11 Chuah et al., 2005 Cocoa shells 33 Meunier et al., 2003 Tree barks 21 Martin-Dupoint et al., 2002 Wheat bran 64 Farajzadeh and Monji, 2004 Sago waste 47 Quek et al., 1998 Barley straw 15.2 Larsen and Schierup, 1981 Hazelnut husk activated Imamoglu and Tekir, 2008 carbon Coconut shell activated carbon Goel et al., 2005 Scanning Electron Microscopy-Energy Dispersion Spectroscopy Analysis Figures 4A and 4B show SEM images of B32 (taken as a representative sorbent) before and after sorption of 400 mg L 1 Pb(II), respectively. Tea leaf surface is full of stoma with a size of approximately 10 μm (Fig. 4A), while the stoma disappeared, likely being covered, after the sorption of Pb(II) (Fig. 4C). The sorption of lead on the sorbent was confirmed by EDS spectra. No lead was identified in the EDS spectra of the pristine sorbent (Fig. 4B), whereas a sharp peak of lead appeared after the sorption (Fig. 4D), showing the presence of lead on the sorbent surface. It can also be seen from Fig. 2. Sorption isotherms of Pb(II) to the tea sorbents. Table 2. Modeling results of the Pb(II) sorption by the tea sorbents. Tea sorbent Langmuir model Freundlich model Q (g kg 1 ) b (L mg 1 ) R 2 K f (g kg 1 )/(mg L 1 ) 1/n n R 2 B ± ± ± ± B ± ± ± ± B ± ± ± ± B ± ± ± ± B ± ± ± ± B ± ± ± ± Liu et al.: Lead Sorption from Aqueous Solutions by Tea Wastes 2263

5 Fig. 3. Fourier transform infrared spectrometer-attenuated total reflectance (FTIR-ATR) spectra of the tea sorbent (B22) after sorption of Pb(II) at 60 mg L 1 for various duration (A) and at various concentrations for 3 h (B). Fig. 4. Scanning electron microscopy (SEM) images and energy dispersion spectroscopy (EDS) spectra of the tea sorbent (B32) before (A and B) and after (C and D) sorption of 400 mg L 1 Pb(II). the two EDS spectra that the peaks of potassium and sodium disappeared and the calcium peak lowered after the sorption, indicating that the ion exchange reactions between Pb(II) and light metal ions (such as potassium, sodium, and calcium) may contribute to the sorption of Pb(II). Tea wastes, as a type of virgin biosorbent, have high contents of carboxyl-light metal/hydrogen complexes (Lim et al., 2008). Heavy metals normally have greater affinities for the carboxyl groups, and thus can readily sorb to the tea sorbents by ion exchange with the light metal/hydrogen ions (Lim et al., 2008). Effect of ph As shown by Fig. 5, Pb(II) sorption by the tea sorbent (B32) increased with increasing ph and leveled off at approximately ph 5. It has been reported that Pb(II) (100 mg L 1 ) tends to hydrolyze and precipitate at ph > 6, making ph effect sorption studies impossible (Doyurum and Celik, 2006; Bulut and Baysal, 2006; Tee and Khan, 1988). Precipitation of the Pb(II) solution was observed at ph > 5.3 in this study. Hence, the optimum ph for Pb(II) sorption fell in the range of 4 to 5. At lower ph, the H + ions would compete with Pb(II) for the surface active sites, and the complex between Pb(II) and the acidic functional groups is expected to be destabilized with the adsorbed Pb(II) released into the solution. Another explanation for the ph-dependent sorption is that at low ph the sorbent surface would be closely associated with hydronium ions (H 3 O + ) which could hinder the access of Pb(II) by electrostatic repulsion to the surface functional groups Journal of Environmental Quality Volume 38 November December 2009

6 Fig. 5. Effect of ph on the sorption of Pb(II) (100 mg L 1 ) by the tea sorbent (B32). Effect of Temperature The effect of temperature on the removal of Pb(II) is shown in Fig. 6. The sorption at 50 C was higher than that at 15 and 25 C, indicating that the sorption of Pb(II) to the tea sorbents could be an endothermic process. Pb(II) sorption was similar at 15 and 25 C, which may be due to the small temperature difference of 10 C in this study (not large enough to influence the sorption). Furthermore, the temperatures during centrifugation (20 min) were all at room temperature, which may also diminish the sorption difference between 15 and 25 C. The higher sorption at higher temperature may be caused by the higher equilibrium capacity of the sorbents, because high temperature may change the sorbent matrix and lead to more sorption sites (Li et al., 2008). Effect of Coexisting Metal Ions Industrial wastewaters normally contain many metal ions of various valences that may compete with and hinder the sorption of the targeted metal ion on the sorbent. Hence, Ag(I), Cu(II), and Al(III) were used to investigate the effect of coexisting cations and their valences on the removal of Pb(II). Removal of Pb(II) decreased with increasing concentration of coexisting metal ions as shown in Fig. 7. At a same concentration, the competitive Fig. 6. Sorption isotherms of Pb(II) by the tea sorbent (B31) at different temperatures. Fig. 7. Effect of metal ions on sorption of Pb(II) (100 mg L 1 ) by the tea sorbent (B31). ability of the coexisting ions increased with increasing ionic valence in an order of Ag(I) < Cu(II) < Al(III). The different effect on the sorption of Pb(II) by the coexisting ions may be explained by the different affinities of metal ions for the sorptive sites on the sorbent surface. It has been pointed out that carboxylic sites are more selective toward multivalent cations than monovalent ones (Shukla et al., 2002). Besides the charge intensity, the ionic radius of the metal ions may also play a role in their competitive abilities by regulating their accessibility to the pore surfaces of the sorbents. The competitive ability of the metal ions decreased with increasing ionic radius in an order of Al(III) (0.535 Å) > Cu(II) (0.73 Å) > Pb(II) (1.19 Å) > Ag(I) (1.26 Å). Conclusions This study showed that tea wastes had a useful potential for heavy metal removal from wastewaters due to their high removal efficiency and sorptive capacity. The sorption was not only dominated by the surface area of tea sorbents. Carboxylic and amine functional groups of the tea sorbents could also play an important role in the binding of Pb(II). Ion exchange with light metal ions (such as potassium, sodium, and calcium) contributed to the sorption of Pb(II) as well. Acknowledgments This work was in part supported by National Natural Science Foundation of China ( , ), National Basic Research Program of China (2008CB418204), Zhejiang Provincial Natural Science Foundation of China (Z507093), and National Key Technology R&D Program of the Eleventh Five-year Plan of China (no. 2006BAJ02A08). References Ahluwalia, S.S., and D. Goyal Removal of heavy metals by waste tea leaves from aqueous solution. Eng. Life Sci. 5: Amarasinghe, B.M.W.P.K., and R.A. Williams Tea waste as a low cost adsorbent for the removal of Cu and Pb from waste water. Chem. Eng. J. 132: Bailey, S.E., T.J. Olin, R.M. Bricka, and D.D. Adrian A review of potentially low-cost sorbents for heavy metals. Water Res. 33: Bulut, Y., and Z. Baysal Removal of Pb(II) from wastewater using wheat bran. J. Environ. Manage. 78: Liu et al.: Lead Sorption from Aqueous Solutions by Tea Wastes 2265

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